1. Field of the Invention
The present invention refers to an improved preform for manufacturing hollow bodies and, in particular, bottles of any kind and size, through a blow-moulding process.
2. Description of Related Art
As is well-known to people skilled in the field, the manufacture of bottles or jars of plastic material today is almost exclusively carried out through a 2-step process which comprises the manufacture and use of hollow, semi-finished products having a substantially cylindrical shape, known precisely as preforms or parisons. In the first step of such process the manufacture of hollow preforms is hence accomplished—in a highly centralised manner, i.e. by few specialised manufacturers—said preforms having great thickness and longitudinal and crosswise size in a ratio of about 1:2 to 1:4 with respect to that of the finished bottle. In the second step of the process—which generally occurs instead in the same plant where bottling of the liquid to be packaged takes place—the hollow preforms are introduced into a mould, heated up to a temperature sufficient to accomplish the necessary softening thereof, stretched to the final length of the bottle by means of a stretching rod introduced in the preform, and hence blow-moulded by introducing one or more flows of compressed air in the heated and stretched preform.
This manufacturing system allows dramatic and various advantages in the entire manufacturing chain, which are well-known to people skilled in the field and which hence do not need to be recalled here, which advantages have led—as it was mentioned earlier—to the almost general adoption, at a global level, of this manufacturing system to achieve hollow bodies and in particular bottles of any kind, shape and size.
In particular, the above-described process has found a particularly suited application in the use of clear plastic materials, such as for example PET (polyethylenterephtalate), which, during the blowing process, undergo such an orientation of the crystalline structure to impart the bottle particularly satisfactory mechanical characteristics, even with very small final thickness values of the material.
Precisely for this type of bottles, a constant search for the optimisation of the manufacturing process has hence developed in order to reduce—the final bottle volume being the same—the quantity of plastic material used and hence the final cost of the bottle. Up until today such search was mainly aimed on the one hand at shape changes of the finished bottle (to create a greater shape stability) and, on the other hand, at the adjustment of the different moulding parameters (stretching ratio of the preform, heating temperature, pressure of the blown air and the like).
Substantially unchanged over time has remained instead the shape of the preform which, as said, consists of a hollow cylindrical body, having a constant or variable thickness, whose open end has already the final shape of the neck area of the bottle—and hence also provided with the necessary threadings for coupling the screw cap—and whose closed end in most cases is semispherical.
In order to reduce the amount of plastic material which, in the finished bottle, remains in the central area of the bottom of the same and hence is of no direct usefulness, preforms with a non-spherical head and, in particular, with a head having a conical, elliptical or parabolic shape have then been suggested and partly used. However, while preforms of this type have actually allowed some advantages during the moulding of the preforms, allowing a modest moulding time reduction thanks to the increased streamlining of the flow path of the melted plastic material (which, as a matter of fact, is injected in correspondence of the vertex of the closed end of the preform), they have not allowed instead appreciable results in terms of reducing the preform weight, since material distribution in the finished bottle did not finally appear significantly different from that of the bottles obtained from preforms having a semispherical head.